1. Field of the Invention
The present invention relates to a method of injection stretch blow molding, which comprises pre-blowing an injection-molded preform of polyethylene terephthalate (PET) and then stretch blow molding it into a bottle in a blow mold to impart heat resistance to the bottle in the same blow mold.
2. Description of the Related Art
Methods of molding PET bottles include a method of injection stretch blow molding, which is referred to as the so-called hot parison system. It comprises quenching a preform to a temperature below the glass transition point at the time of injection molding; releasing the preform from an injection mold and an injection core while surface layers inside and outside the body of the preform are semi-cured as a result of the quenching time and temperature and the inside is still at a higher temperature; and stretch blow molding the preform before the surface temperature of the preform reaches the peak temperature, thereby molding a bottle with a thin wall body.
The hot parison system may include a means for molding heatproof bottles, which comprises transferring a high-temperature mold-released preform in an injection station to a temperature regulating station; transferring the temperature-regulated preform to an stretch blow molding station; stretch blow molding a hollow molded product (bottle); transferring the bottle to a secondary processing station; and blowing high-temperature air into the bottle for heat treatment in a heat treatment mold.
A temperature regulating means may be achieved through a method, which comprises housing a high-temperature mold-released preform in a cooler temperature regulating mold; and expanding only a body of the preform by pre-blowing to uniform the wall thickness and temperature of the body. Another temperature regulating means comprises providing a temperature regulating mold and a blow mold in parallel; shifting both the molds alternately relative to the preform to perform adjustment of the preform by pre-blowing and stretch blow molding of the bottle.
[Patent Document 1]JP 2902119 B[Patent Document 2]JP 60-247541A[Patent Document 3]JP 58-208020A[Patent Document 4]JP 58-194521A
In the above hot parison system, the injection-molded preform is released from the injection mold and the injection core while surface layers inside and outside the body of the preform are semi-cured and the inside is still at a higher temperature. Accordingly, there is an extremely uneven difference in temperature between the inner center of the body of the preform and the surface thereof as graphed in FIG. 9. In addition, temperature distributions across the body in section exhibit mountain shapes with high-temperature inner portions as shown in FIG. 10. FIGS. 9 and 10 show measured surface temperatures and inner temperatures simulated from the measured surface temperatures. In such the body temperatures, the inner temperature descends as heat radiation from the surface with the passage of time reduces the inner thermal energy. In contrast, the surface temperature ascends as the surface layer is heated from inside. This relative temperature variation reduces the temperature difference and the temperature distribution also varies from a higher mountain shape t1 to a lower mountain shape t2 and to a much easier hill shape t3 to achieve evenness. The time for achieving a temperature balance is longer, however, and a certain temperature difference is retained even after the surface temperature reaches the peak temperature.
In stretch blow molding before the outer surface temperature of the preform reaches the peak temperature, temperatures inside and outside the body are uneven and the temperature difference causes differences in crystal density in the lateral section of the body of the molded bottle. In this case, the crystal density is higher in the surface layer which is lower in temperature than the inner center and is located in the crystal temperature region. Therefore, the surface layer is advantageous to provide a good quality bottle, excellent in surface brightness and drop strength. The drop strength of the bottle can be improved as the thickness of the surface layer is increased. In this case, the increase in surface layer thickness results in a reduced high-temperature region at the inner center and a lowered amount of accumulated heat. Accordingly, formation of the surface layer by cooling has a limit. In addition, the distribution of crystal densities may make it easy to peel off the surface layer. Therefore, a temperature regulating means is applied to evenly adjust the inner and outer temperatures of the body of the preform.
The stretch blow molded bottle by the hot parison system is lower in crystal density than that by a cold parison system and has a problem in heatproof processing because of the uneven distribution of densities. Therefore, the high-temperature mold-released preform is adjusted evenly and then stretch blow-molded into a bottle. Thereafter, the bottle is heat-treated into a heatproof bottle. Therefore, in the hot parison system, molding of the heatproof bottle requires a longer time and the molded bottle is heatproof processed in a secondary processing. Accordingly, it has problems associated with poor yields and higher costs.